Nickel chromium iron alloys

ABSTRACT

NICKEL-CHROMIUM-IRON ALLOYS OF SPECIAL COMPOSITION AND CONTAINING CARBON, MOLYBDENUM, TITANIUM, ALUMINUM, BORON, ETC., ARE PARTICULARLY SUITABLE FOR WROUGHT ROTOR DISCS FOR GAS TURBINE ENGINES AND FOR OTHER ARTICLES AND PARTS THAT ARE SUBJECTED IN USE TO HIGH STRESS AT TEMPERATURES UP TO ABOUT 700*C., E.G., 550*C. TO 700*C.

United States Patent 70 Int. Cl. C22c 19/00, 39/20 US. Cl. 75-122 6 Claims ABSTRACT OF THE DISCLOSURE Nickel-chromium-iron alloys of special composition and containing carbon, molybdenum, titanium, aluminum, boron, etc., are particularly suitable for wrought rotor discs for gas turbine engines and for other articles and parts that are subjected in use to high stress at temperatures up to about 700 C., e.g., 550 C. to 700 C.

During the course of the last thirty years or so, the metallurgical art has witnessed a considerable number of significant accomplishments in respect of the mechanical and other properties characteristic of alloys used for high temperature purposes. And there seems to be little doubt that this has been brought about in no small measure by the more stringent demands imposed by more severe operating conditions, new designs, etc.

However, in the development of new alloys capable of meeting the challenges of higher temperature, greater speed, less weight, or whatever the service requirements, it is not uncommon to find that as the minimum specification as to one or more properties, say, stress-rupture strength, is met some other necessary characteristic is adversely affected or is otherwise found wanting. That is to say, providing for a combination of different but indispensible characteristics is fraught with difficulty since the very requirements of various properties are often inherently self-conflicting; for example, hardness and machinability, strength and ductility, etc. Adding to such difficulties are those applications where a unitary fabricated component must exhibit different combinations of properties at different sections thereof, the rotor disc being illustrative.

With regard to rotor discs, the different properties required are indeed manifold and complex. Contributing to this and of particular significance is the large variation of temperature occurring radially between the center or hub and the periphery or rim of the disc. This temperature gradient is accompanied by a stress gradient in the opposite sense so that the highest stress occurs in the lowtemperature region near the hub and vice versa. A rotor disc material must therefore not only have a high creep strength up to the highest temperatures to which it is exposed to ensure freedom from distortion by creep in service, particularly at the rim, but also a high proof stress and ultimate tensile strength at more moderate temperatures to ensure that the high hub stresses do not lead to distortion or fracture on loading. It must have adequate tensile ductility and must not be notch-sensitive at temperatures corresponding to that at which the rim, with its fir-tree recesses for the turbine blades, operates. Furthermore, the need to produce a large and relatively complex shape requires that the alloy shall be hotworkable.

As is known, nickel alloys have found extensive use in the production of rotor discs as well as a host of other diverse high temperature products. Typical of the relatively lower-cost alloys hitherto used for rotor discs is the nickel-chromium-iron alloy (herein designated Alloy A) having the following approximate nominal composi- 3,758,295 Patented Sept. 11, 1973 ICC tion: carbon 0.05%, chromium 12.5%, nickel 42.5%, molybdenum 5.7%, titanium 2.8%, aluminum 0.2%, boron 0.015%, balance iron and impurities. While this alloy has a very satisfactory combination of properties, the progressive increase in the designed operating temperatures and stresses of gas-turbine engines calls for a material having a substantially higher proof stress. This desideratum can be obtained by the use of nickelchromium-base alloys containing high contents of such alloying elements as molybdenum and niobium, but such alloys have the serious disadvantages of being very difficult to hot-work and extremely difficult to machine.

Our United Kingdom specification No. 1,132,724 describes alloys having improved proof stress and yet are still readily hot-workable and machinable, the alloys containing about 0.02 to 0.1% carbon, 11% to 16% chromium, 4% to 7% molybdenum, 0.3% to 0.8% niobium, 2.0% to 3.5% titanium, 0.25% to 0.75% aluminum, 35% to 45% nickel, 0.003% to 0.02% boron, and up to 0.1% zirconium, the balance, apart from impurities, being iron. However, although these alloys possess improved proof stress over the above-mentioned Alloy A, the tensile and impact strengths thereof at temperatures of from 650 C. to 700 C. may still not be adequate for some applications.

It has now been discovered that a good combination of the overall properties above-discussed can be achieved relatively economically with certain novel alloys containing correlated percentages of nickel, chromium, iron, carbon, molybdenum, titanium, aluminum and boron.

It is an object of the present invention to provide new and improved nickel-chromium-iron alloys.

It is a further object to provide alloys capable of use as wrought rotor discs and other structural (and also cast) products.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention contemplates providing alloys containing (in weight percent) about 0.02% to about 0.1% carbon, about 10% to about 15% chromium, about 5% to about 7% molybdenum, about 3.3% to about 4.5% titanium, about 0.2% to about 1.1% aluminum, about 45% to about 55% nickel, about 0.003% to 0.02% boron, the balance being essentially iron. It is particularly advantageous in seeking an optimum combination of properties that the total percentage of titanium plus aluminum is (a) greater than 0.06 times the percentage of nickel plus 1.0 and (b) less than 0.24 times the percentage of nickel minum 6.0.

In carrying the invention into practice, it is important that the content of each of the constituents of the alloys responds to the ranges above set forth. With percentages of carbon significantly less than 0.02% the alloys are notch-sensitive, their life in stress-rupture tests being greatly reduced by the presence of notches. Too much carbon, on the other hand, leads to the formation of excessive amounts of carbides which tend to segregate and give rise to directionality effects in the tensile and stress-rupture properties of the wrought material. Hence, the carbon content should not exceed about 0.1% and beneficially is not more than 0.08%. It is most advantageous that the carbon be at least 0.03% but not more than 0.06%.

At least about 10% chromium is required for adequate resistance to oxidation at operating temperature, but more than about 15% renders the alloys liable to embrittlement on prolonged heating.

Molybdenum contributes to the stress-rupture strength of the alloys, and at least 5% is required for this purpose. Raising the molybdenum content much above 7%, however, makes the alloy very difficult to work and also susceptible to embrittlement, and preferably the molybdenum content is from 5.4% to 6.7%.

Should the titanium be less than about 3.3%, it can be expected that one or more of proof stress, hardness or stress-rupture strength will be inadequate. However, increasing the titanium content above 4.5% undesirably reduces the ductility. It is much preferred that the titanium be at least 3.5% and not more than 4.25%.

Aluminum in an amount of about 0.2% or more is required to avoid the risk of embrittlement. Surprisingly, however, increasing the aluminum content, while increasing the tensile ductility of the alloys, is found to lower their proof stress. For this reason it should not exceed about 1.1%. An aluminum range of 0.3% to 0.9% is deemed quite satisfactory but in striving for the best combination of proof stress and ductility it is advantageously from 0.5% to 0.7%.

The alloys should contain at least 45% of nickel to avoid instability and embrittlement through the formation of Laves phase on prolonged heating. Too much nickel, however, tends to reduce the proof stress and unduly raise the cost of the alloy, and the nickel content must therefore not exceed 55%. Moreover, as above indicated, it is to considerable advantage that the percentage of nickel, titanium and aluminum be correlated to satisfy the relationships designated as (a) and (b) above. By observing these relationships an exceptionally good combination of desired mechanical characteristics is more consistently attained.

A particularly satisfactory combination of properties is exhibited with alloys of 0.03% to 0.08% preferably 0.03% to 0.06%, carbon, 11% to 14% chromium, 5.4% to 6.7% molybdenum, 3.75% to 4.1% titanium, 0.5% to 0.7% aluminum, 47% to 52% nickel, 0.01% to 0.02% boron, the balance essentially iron.

To develop the best combination of proof stress and tensile ductility at elevated temperature the alloys should be solution heated and then aged in one or more stages. Advantageously, solution heating comprises heating at 1040 C. to 1100 C. for 1 to 8, preferably for 2 to 4 hours. A single aging treatment at 700 C. to 760 C. for at least 8 hours, preferably at 730 C. for 16 hours, is preferred. Alternatively, the alloys may be aged in two stages, the first consisting of heating at 740 C. to 780 C. for more than 1 hour followed by aging at 675 C. to 720 C. for at least 8 hours. One suitable aging treatment of this kind comprises heating at 775 C. for 20 hours and then at 700 C. for 16 hours. Cooling after the solution heating is preferably rapid, e.g., a water quench, but cooling between aging treatments may be conducted in air.

The present alloys, after solution heating for 2 hours at 1095 C., water quenching, aging for 20 hours at 775 C., air cooling, aging at 700 C. for 16 hours and air cooling, are generally characterized by a proof stress (0.1% offset) at 600 C. of at least about 62 t.s.i. (long tons per square inch), a tensile ductility of at least about 14% elongation and a stress-rupture life of over 100 hours under a stress of 47 t.s.i. at 650 C. They have excellent hot workability, can be deformed hot by rolling, forging, swaging and extrusion and can be deformed cold by rolling, drawing and swaging.

For the purpose of giving those skilled in the art a better appreciation of the invention, the following illustrative data are given:

Three alloys A, B and 1 were prepared by air-melting and vacuum-refining followed, in the case of Alloys A and B, by electroslag refining. Ingots cast from the alloys were forged to cheeses and then to rotor disc blanks 14 inches in diameter having a central punched-out bore 3 inches in diameter. After heat treatment to develop the optimum tensile properties, the disc blanks were sectioned and test pieces were machined from similar positions adjacent and tangential to the central bore.

Alloy A had the nominal composition set forth above,

and Alloy B was a preferred alloy of our above-mentioned specification No. 1,132,724 having the following nominal composition, in percent by weight: carbon 0.04%, chromium 12.5%, nickel 42.5%, molybdenum 5.75%, titanium 3.0%, aluminum 0.4%, niobium 0.55% and boron 0.015%, the balance, apart from impurities which included silicon 0.2% and manganese 0.1%, being iron. Alloy No. 1 represents a preferred alloy of the subject invention having the nominal weight percentage composition: carbon 0.03%, chromium 12.5%, titanium 3.9%, aluminum 0.6%, nickel 49%, molybdenum 6.0%, boron 0.015%, balance iron, apart from impurities, Alloy A was given a commercially recommended heat treatment comprising solution heating for 2 hours at 1095 C., water quenching, and aging for 2 hours at 775 C. and then for 24 hours at 705 C., with intermediate and final cooling in air. Alloy B was solution heated for 4 hours at 1060 C., water quenched and then aged for 16 hours at 730 C. and air-cooled. Alloy No. 1 was solution heated for 2 hours at 1095 C., water quenched, aged at 775 C. for 20 hours, air-cooled, aged at 700 C. for 16 hours and air-cooled.

The results of tensile tests are set forth in Table I.

TABLE I Proof stress 0. 1% 0. 2% Tensile Elongaoflset ofiset stress tion (t.s.i.) (t.s.i.) (t.s.i.) (percent) The generally advantageous effect on the tensile properties of the higher titanium and nickel contents of Alloy No. 1 is surprising, since from theoretical considerations it might have been expected that the higher nickel content would reduce the proof stress and the higher titanium content would substantially reduce the ductility.

While the invention specifically includes wrought rotor discs for gas-turbine engines made from alloys in accordance herewith, the alloys are also suitable for use in the production of components and parts in gas-turbines generally, and also for jet engines and other applications requiring a good combination of stress rupture strength, proof stress, tensile strength, workability, machinability, etc., all at low cost. It is contemplated that the alloys may also be used for bolting, fittings, blades, buckets, nozzles, afterburner parts, shrouding, extruded products, etc. The alloys can be produced in the form of plate, sheet, strip, rod, wire, bar, tubing, forgings and other mill forms. In addition to wrought products, the alloys can also be used in the form of castings.

In referring to iron as constituting the balance or essentially the balance of the subject alloys, the presence of other elements, as will be appreciated by those skilled in the art, is not excluded such as those commonly present as incidental elements, e.g., deoxidizing and cleansing constituents, and impurities ordinarily associated therewith in amounts that do not adversely affect the basic characteristic of the alloys. As to silicon and manganese, common impurities in such alloys, it is not necessary that either should exceed 0.5 each and, in fact, they should be held to low levels. Not more than traces or very little lead and sulfur should be present but the alloys can contain up to at least 0.5% copper, 1% cobalt and 0.1% zirconium. However, it is important that they be free or substantially free of niobium or tantalum, the combined percentage of which should be held to less than about 0.3%, e.g., less than 0.2%.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand.

Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A nickel-chromium-iron alloy characterized by (a) a proof stress of at least about 62 t.s.i. at a temperature of about 600 C., (b) a tensile ductility of at least 14% at 650 C., (c) a stress rupture-life of at least 100 hours under a stress of 47 t.s.i. at a temperature of about 650 C. and (d) good hot workability, said alloy consisting of about 0.02% to about 0.1% carbon, about to about chromium, about 5% to about 7% molybdenum, about 3.3% to about 4.5 titanium, about 0.2% to 1.1% aluminum, the total percentage of the titanium plus aluminum being greater than 0.06 time the percentage of nickel plus 1.0 and less than 0.24 time the percentage of nickel minus 6.0, about 45% to about 55% nickel, about 0.003% to about 0.02% boron, up to about 0.5% each of silicon, manganese and copper, up to 1% cobalt, up to less than 0.3% of columbium plus tantalum, and the balance essentially iron.

2. An alloy as set forth in claim 1 and which contains about 0.03% to about 0.06% carbon, about 11% to 14% chromium, about 5.4% to 6.7% molybdenum, about 3.75% to 4.1% titanium, about 0.5 to 0.7% aluminum, about 47% to 52% nickel, about 0.01% to about 0.02% boron, the balance being essentially iron.

3. As a new article of manufacture, a fabricated or cast aircraft component formed from an alloy composition as set forth in claim 1.

4. A wrought rotor disc produced from an alloy composition as set forth in claim 1.

5. A nickel-chromium-iron alloy characterized by (a) a proof stress of at least about 62 t.s.i. at a temperature of about 600 C., (b) a tensile ductility of at least 14% at 65 0 C., (c) a stress rupture life of at least about 100 hours under a stress of about 47 t.s.i. at a temperature of about 650 C. and (d) good hot workability, said alloy consisting of about 0.02% to about 0.1% carbon, about 10% to about 15% chromium, about 5% to about 7% molybdenum, about 3.5% to about 4.5% titanium, about 0.3% to about 0.9% aluminum, about to about nickel, about 0.003% to about 0.02% boron, up to about 0.5% each of silicon, manganese and copper, up to 1% cobalt, up to less than 0.3% of columbium plus tantalum, and the balance essentially iron.

6. An alloy in accordance with claim 5 in which the nickel content is from about 47% to about 52%.

References Cited UNITED STATES PATENTS 3,048,485 8/1962 Bieber 171 RICHARD 0. DEAN, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent 3,758,295 Dated September 11, 1973 Inventor) MICHAEL MORLEY and ALLAN ROY K'NOTT It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 49, for "minum" read "minus" Column 3,' line 25, for "age" read "ages" Signed and sealed this 6th day of August 197A.

(SEAL) Attest:

C. MARSHALL DANN Commissioner of Patents McCOY M. GIBSON, JR. Attesting Officer 

